ASHRAE Journal - January 2013 - 74

engineer’s notebook
17 23 Insulation Level (R-Value) 28 34 40 45 51 57

Figure 1: Heating load as function of levels of insulation and southfacing window sizes.

6000 5000 4000 3000 2000 1000 0

Heating Load (kW)

Window 35% of South Façade Window 50% of South Façade Window 25% of South Façade

A
3 4 5 6 7 8 Insulation Level (RSI) 9 10

Two Shading Façades

one-half requires 9% less heating (due to envelope heat transfer) than an L-shape with DR of 1 (and about 7% more than the rectangle of same plan area). Number of shading façades: Increasing the number of shading façades reduces heat gain due to self-shading. The effect of increased number of shading facades can be mitigated by managing depth ratio and wing angles. Angle between wings: By increasing the angle enclosed between shading and shaded façades, self-shading can be controlled.

Roof Design
It is advisable to design roof geometry for potential installation of solar collectors. Energy generation potential of building integrated photovoltaic (BIPV) is affected by the tilt angle, orientation and area of solar-facing roof surfaces, as well as by shading from adjacent surfaces (and/or buildings). Tilt angle: An optimal tilt angle of roofs for integration of PV approximates the latitude of the location, which represents a compromise between summer optimal tilt angle (latitude –15°) and winter optimal tilt angle (latitude +15°). 4 Orientation: In mid-latitude location, rotation of a roof with 45° tilt angle by up to 45°, relative to south, results in some 5% reduction of annual electricity generation. A rotation of the system by 60°, west or east of south, results in a reduction of some 12%.4 The implementation of BIPV with different orientations enables shifting of the timing of peak generation relative to solar noon. Time spread of peak electricity generation can result in a more even electrical generation profile, thus imposing less demand on the electric grid. This is economically beneficial in regions where the cost and pricing of electricity vary with time of day. Surface area: South-facing roof area of a given floor area can be managed by manipulation of the aspect ratio and/ or of shape design, as demonstrated in L variations (Figure 2c), where rotation of one wing leads to enlargement of the south-facing roof surface.
b b a

HVAC&R Research offers comprehensive reporting of research in the fields of the built environment and cooling technologies, including underlying thermodynamics, fluid dynamics and heat transfer. Included in the ISI Web of Science and Current Contents databases, this bimonthly journal is ASHRAE’s archival research publication. Quick Access to New Articles Through the Accepted Manuscripts Online Initiative, accepted, uncorrected versions of manuscripts, fully citable , and assigned a DOI® are available online within seven days of receipt.

Figure 2: (A) Illustration of the number of shading facades, (B) Depth ratio, (C) L variants with different angles between shading facades, (D) application in neighborhoods.
2. Hachem, C., A. Athienitis, P. Fazio. 2012. “Evaluation of energy supply and demand in solar neighbourhoods.” Journal of Energy and Buildings 49(6):335–347. 3. Chiras, D. 2002. The Solar House: Passive Heating and Cooling. White River Junction, VT: Chelsea Green Publishing. 4. Hachem C., A. Athienitis, P. Fazio. 2012. “Design of roofs for increased solar potential of BIPV/T systems and their applications to housing units.” ASHRAE Transactions 118(2).

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References
1. Hachem, C., A. Athienitis, P. Fazio. 2011. “Parametric investigation of geometric form effects on solar potential of housing units.” Solar Energy 85 (9):1864–1877.

Caroline Hachem is a postdoctoral fellow and lecturer, and Andreas Athienitis, P.E., is a professor at the Department of Building, Civil and Environmental Engineering at Concordia University, Montreal.
January 2013

ASHRAE Journal

ASHRAE Journal - January 2013

Table of Contents for the Digital Edition of ASHRAE Journal - January 2013